High-resolution OH maser observations of IRC + 10420

Abstract

The 1612-MHz hydroxyl maser emission of the infrared star IRC + 10420 was studied over a 10-month period both with a very long baseline interferometer yielding an angular resolution of approximately 0.10 arcsec and with single telescopes. The 1612-MHz VLBI data indicate that individual features are highly circularly polarized (typically 50%), with linear sizes from less than 100 to 400 AU. A map of the OH features of the +44 to +48-km/s complex in left circular polarization indicates a disklike structure of overall extent 8000 AU. The single-telescope spectra also show circular polarization (upper limit about 15%), with time variability of the total flux over a period of several months. It is suggested that IRC + 10420 is a massive star moving rapidly to the left on the horizontal branch of the H-R diagram and which has left a 'fossil remnant' expanding cloud containing the hydroxyl emission. Comparison with other OH/IR supergiant stars indicates that previous distance estimates of 6.8 kpc for IRC + 10420 are too large by a factor of 2.